The oligodynamic effect is the term given to the ability of small amounts of heavy metals to exert a lethal effect on bacteria (from the Greek: oligos, small; dynamis, power). The effectiveness of heavy metals as antimicrobials is believed to be due to the high affinity of cellular proteins for metallic ions. Bacteria cells die due to the cumulative effects of ions within the cell, even if the concentration of ions in a solution is miniscule. Metals that generally show a strong oligodynamic effect are (in order of decreasing strength) Hg>Ag>Cu>Zn>Fe>Pb>Bi. Among these metals, silver and zinc have been used in materials for various applications and industries, such as materials for use in medical devices, food processing products, textiles, and sanitary ware. Oligodynamic elements other than silver and zinc, either due to human toxicity or some incompatibility with the intended matrix material (e.g. changes in color), are rarely used as antimicrobial agents in material applications. Compared to zinc, silver and its salts exert a much stronger antimicrobial effect against common bacteria such as Staphylococcus Aureus and Escherichia coli. Zinc oxide, however, generally shows much better efficacy than silver against various fungi. Another practical factor from a manufacturing standpoint is that silver is far more expensive than zinc, with a market price over 100 times greater per unit weight compared to zinc.
U.S. Pat. Nos. 5,807,641 and 5,882,808 relate to antimicrobial sanitary ware produced by adding silver compounds to the ceramic glaze layer. The silver is added to the glaze slurry as a salt or oxide. The glaze slurry is applied to the ceramic body and fired at a temperature generally exceeding 1100° C. This approach can provide good antibacterial efficacy, but in practice, the level of silver required to obtain this effect results in an unacceptably large increase in the cost of the glaze. For example, a typical toilet and tank combination contains about 6.5 lbs of glaze. Due to the relatively high vapor pressure of silver and its compounds at temperatures above 1200° C., at least 2 wt % of an antimicrobial silver compound is needed to impart strong antimicrobial efficacy to the fired sanitary ware body. The cost of antimicrobial silver compounds is roughly $100/lb, which at 2% loading results in an added cost of $13 per toilet and tank combo. This cost requires a price increase for antimicrobial sanitary ware that is well beyond what many consumers in the Americas and Europe are willing to pay for the feature. Additionally, a large part of the silver in the glaze vaporizes and condenses on the walls of the kiln, which over time can build up to troublesome levels and result in manufacturing downtime, thereby further increasing the cost of manufacturing these pieces. Thus, there is a need for a more cost effective means than using silver for producing antimicrobial sanitary ware for these markets.
Of the other metals that have strong oligodynamic effects, zinc is most suited for use in sanitary ware applications. Mercury, lead, and bismuth present toxicity and/or environmental issues, whereas iron and copper compounds would eliminate the possibility of producing white pieces. Zinc oxide is already used as a flux material in some sanitary ware glaze systems, albeit at levels that are too low to yield any significant antimicrobial effect. For example, Japanese Patent Application 10-227686 relates to an antimicrobial glaze formulation that contains 6-20 wt % of zinc compounds measured as zinc oxide. The inventors state that at least 6 wt % of zinc compounds is necessary to obtain consistent antimicrobial efficacy. Using such a large amount of zinc in a sanitary ware glaze system, although it might provide antimicrobial properties at a cost much lower than that obtainable through the use of silver compounds, presents manufacturing issues that severely limit the practicality of this approach. Such a sanitary ware glaze system having more than 6 wt % zinc oxide, will begin to suffer severe pitting and surface irregularity defects. These defects become even more severe if the circulation of air in the kiln is not sufficient.
Similar needs and issues exist for antimicrobial porcelain enamel systems. Whereas glazes are glass coatings applied to ceramic substrates (bodies), porcelain enamel is the general term applied to such glass coatings on metallic substrates. For example, porcelain enamels are widely applied to steel and cast iron bodies in the manufacture of sinks, bathtubs, hot water heaters, cookware, and some appliances.